2 tablespoons canola oil, plus more for pan
4 cups all-purpose flour, plus more for pan
1/2 cup granulated sugar
2 1/2 teaspoons baking powder
1/2 teaspoon salt
1 large egg
1/2 cup milk
2 teaspoons pure vanilla extract
1 cup packed light-brown sugar
1 1/2 teaspoons ground cinnamon
1 cup (2 sticks) unsalted butter, melted and cooled
Confectioners' sugar, for dusting
Place rack in center of oven, and heat oven to 325 degrees. Lightly brush a 9-by-12 1/2-inch baking pan with canola oil, dust with flour, and tap to remove excess. Set aside. In a medium bowl, sift together 1 1/2 cups flour, granulated sugar, baking powder, and salt; set aside. In a second bowl, whisk together egg, milk, canola oil, and vanilla. Using a rubber spatula, fold dry ingredients into egg mixture.
Spread batter evenly into prepared pan, and set aside. In a medium bowl, combine remaining 2 1/2 cups flour, brown sugar, and cinnamon. Pour melted butter over flour mixture, and toss with a rubber spatula until large crumbs form. Sprinkle crumbs over batter.
Transfer pan to oven, and bake, rotating pan after 10 minutes. Continue baking until a cake tester comes out clean, about 10 minutes more.
Transfer baking pan to a wire rack to cool. Dust with confectioners' sugar. Using a serrated knife or bench scraper, cut into 3-inch squares. Store in an airtight container for up to 3 days.